<!DOCTYPE html>
<html>
<head lang="en">
    <meta charset="UTF-8">
    <title></title>
    <style>
        canvas{border: 1px solid red;}
    </style>
</head>
<body>
<canvas id="box" width="800" height="800"></canvas>
<script>
    var box=document.getElementById("box");
    var pen=box.getContext("2d");
    pen.translate(400,400);
//    pen.beginPath();
//    pen.arc(0,0,50,0,Math.PI*2,true);
//    pen.closePath();
//    pen.stroke();

//    pen.beginPath();
//    pen.arc(0,0,100,0,Math.PI*2,true);
//    pen.stroke();
//    pen.closePath();
for(var i=100;i>=20;i-=10){
    pen.beginPath();
    var a=Math.floor(Math.random()*255);
    var b=Math.floor(Math.random()*255);
    var c=Math.floor(Math.random()*255);
    pen.arc(0,0,i,0,Math.PI*2,true);
    pen.fillStyle="rgb("+a+","+b+","+c+")";
    pen.fill();
    pen.closePath();
    pen.stroke();

}



</script>
</body>
</html>